One of the great unsolved mysteries regarding neurobiology of mood disorders is - why do some patients suffer only from episodes of depression (unipolar depression or UD, also known as major depression) while others suffer from episodes of both depression and the opposite mood state of mania (bipolar disorder or BD)? In the clinical setting, both UD and BD depression (BDD) are difficult to differentiate and can only be teased apart by obtaining a detailed longitudinal history to identify periods of (hypo) mania. However, in young patients, early in the course of the illness, this history is either very difficult to elicit or not present because the first few episodes of BD are usually of depression. Only with time, and usually after years of misdiagnosis and inappropriate treatment, a proportion of the depressed subjects start suffering from episodes of overt (hypo)mania and reveal themselves to be actually suffering from BD. Therefore, there is a critical need to identify, particularly in young patients, neurobiological differences between BDD and UD as well as differences between UD patients at a high risk for developing BD (HRUD) and UD patients with low risk of developing BD (LRUD). The investigators have been conducting functional magnetic resonance imaging (fMRI) studies of corticolimbic connectivity and activity for nearly a decade and in recent studies have identified resting state connectivity abnormalities as well as task-induced activation abnormalities in BD and UD. Preliminary data suggests that these measures may be helpful in differentiating between BDD and UD. This proposal will investigate corticoamygdalar connectivity and activation abnormalities in 40 BDD, 120 UD (60 HRUD and 60 LRUD) young patients (16 - 25 yrs of age) and 40 closely matched healthy controls (HC). Besides, investigating cross-sectional differences between groups, an exploratory prospective validation of the BDD related abnormalities would also be conducted by treating the HRUD and LRUD subjects with antidepressants for 24 months with follow-up assessments at regular intervals. Subjects will be assessed periodically for emergence of (sub)threshold symptoms of (hypo)mania. The relationship between baseline imaging measures and development of (sub)threshold symptoms of (hypo)mania or frank conversion to a bipolar diagnosis will be investigated. Innovative aspects of this proposal are: it addresses the understudied area of difference between unmedicated BDD and UD patients using fMRI measures to study circuit level abnormalities; studies young patients in which the issue is most important; uses a cross-sectional as well as prospective study design; and in line with aims of the NIMH Research Domain Criteria (RDoC) project investigates: 1)the similarities between HRUD and BDD subjects; and 2) the relationship between baseline imaging measures and the development of (hypo)mania using a continuous measure of increase in symptoms rather than only as a dichotomous measure based on DSM-IV diagnosis. This study will have a critical impact both on the understanding of the neurobiology of mood disorders as well as on how to differentiate between young BDD and UD patients.